Human monocytes are activated by endotoxin and other inflammatory microbial products to produce hormone-like factors such as Interleukin 1 (IL-1). We have purified human IL-1 from peripheral blood monocytes to homogeneity using ultrafiltration, phenyl Sepharose chromatography, Ultrogel AcA54 chromatography and preparative isoelectricfocusing. Using purified samples of IL-1, we have determined that IL-1 is the mediator which activates chondrocytes to degrade cartilage matrix. We have also identified and cloned a human monocytic leukemia cell line (THP-1) which produces one of the two major components of IL-1. This cell line also produces a novel suppressor factor which inhibits the production and the action of Interleukin 2.